Re: [PATCH v2 1/8] mm: page_alloc: optimize pfn_range_valid_contig()

[Dev Jain <dev.jain@arm.com>]

On 18/09/25 6:49 pm, Kefeng Wang wrote:
> The alloc_contig_pages() spends a lot of time in pfn_range_valid_contig(),
> we could check whether the page in this pfn range could be allocated
> before alloc_contig_range(), if the page can't be migrated, no further
> action is required, and also skip some unnecessary iterations for
> compound pages such as THP and non-compound high order buddy, which
> save times a lot too. The check is racy, but the only danger is skipping
> too much.
>
> A simple test on machine with 116G free memory, allocate 120 * 1G
> HugeTLB folios(107 successfully returned),
>
>    time echo 120 > /sys/kernel/mm/hugepages/hugepages-1048576kB/nr_hugepages
>
> Before: 0m2.124s
> After:  0m0.602s
>
> Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
> ---
>   mm/page_alloc.c | 20 ++++++++++++++++++--
>   1 file changed, 18 insertions(+), 2 deletions(-)
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 478beaf95f84..5b7d705e9710 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -7012,6 +7012,7 @@ static bool pfn_range_valid_contig(struct zone *z, unsigned long start_pfn,
>   {
>   	unsigned long i, end_pfn = start_pfn + nr_pages;
>   	struct page *page;
> +	struct folio *folio;
>   
>   	for (i = start_pfn; i < end_pfn; i++) {
>   		page = pfn_to_online_page(i);
> @@ -7021,11 +7022,26 @@ static bool pfn_range_valid_contig(struct zone *z, unsigned long start_pfn,
>   		if (page_zone(page) != z)
>   			return false;
>   
> -		if (PageReserved(page))
> +		folio = page_folio(page);
> +		if (folio_test_reserved(folio))
>   			return false;
>   
> -		if (PageHuge(page))
> +		if (folio_test_hugetlb(folio))
>   			return false;
> +
> +		/* The following type of folios aren't migrated */
> +		if (folio_test_pgtable(folio) | folio_test_stack(folio))
> +			return false;
> +
> +		/*
> +		 * For compound pages such as THP and non-compound high
> +		 * order buddy pages, save potentially a lot of iterations
> +		 * if we can skip them at once.
> +		 */
> +		if (PageCompound(page))
> +			i += (1UL << compound_order(page)) - 1;

Can we instead do
if (folio_test_large(folio))
     i += folio_nr_pages(folio) - folio_page_idx(folio, page).

> +		else if (PageBuddy(page))
> +			i += (1UL << buddy_order(page)) - 1;
>   	}
>   	return true;
>   }